Mechanistic Investigation on 1,5-to 2,6-Dimethylnaphthalene Isomerization Catalyzed by Acidic β Zeolite: ONIOM Study with an M06-L Functional

The isomerization of 1,5- to 2,6-dimethylnaphthalene (DMN) over beta zeolite has been investigated by applying a newly developed density functional named M06-L, incorporated into the ONIOM scheme: M06-L/6-31G(d,p): UFF. Two consecutive reaction mechanisms over the extended zeolite framework have been carefully examined: the 1,5- to 1,6-DMN isomerization followed by the 1,6- to 2,6-DMN isomerization. Both catalytic processes take place via the same mechanism. The isomerization process starts from the protonation of the DMN step, creating the naphthalynic carbocation. Subsequently, the intramolecular methyl shift occurs from the alpha-position to the adjacent beta-position of the naphthalynic carbocation. In the final step, the anionic zeolite framework takes a proton away from the naphthalynic carbocation, yielding a desired 1,6-DMN or 2,6-DMN molecule. The methyl migrations are the rate-determining steps and require activation barriers of 25.69 and 21.05 kcal/mol for the 1,5- to 1,6-DMN and 1,6- to 2,6-DMN processes, respectively. The calculated reaction profiles are in agreement with the experimental prediction that the 1,5- to 1,6-DMN isomerization is the kinetically controlled step. The results in this study show the excellent performance of a combination of the newly developed functional and the confinement effect represented by the universal force field (M06-L/6-31G(d,p): UFF) for investigating the trans formations of aromatic species in the zeolite system.